Optical sensors such as standard Fibre Bragg Gratings (FBG) are known from prior art. Generally such sensors are interrogated with polarized frequency swept sources (e.g. polarized tunable lasers) and suffer from polarization dependent frequency shift (PDFS) problems, typically in the range of 1-4 pm depending on the technique used to write the FBG and the birefringence induced when packaged in a transducer.
A type of FBGs, called birefringent FBGs, (Bi-FBG), are known from prior art. Such Bi-FBG are known to have polarization dependent responses to an interrogation signal. Examples of Bi-FBGs, such as polarization maintaining FBGs (PM-FBG), are disclosed in published papers by G. Chen, et al. “Simultaneous strain and temperature measurements with fibre Bragg grating written in novel hi-bi optical fibre”, IEEE Photonics Technology Letters 2004, 16(1), p 221-223] and micro-structured FBGs (MS-FBG)] and S. Sulejmani, et al. “Control over the pressure sensitivity of Bragg grating-based sensors in highly birefringent microstructured optical fibres”, IEEE Photonics Technology Letters, 2012, 24(6), p 527-529].
Bi-FBGs that provide a response to at least two different physical variables are known from prior art, for example, PM-FBG (temperature and strain) or MS-FBG (temperature and pressure).
PM-FBG sensors can provide strain-independent temperature measurements and/or self-compensating strain measurements without a requirement for an extra temperature FBG, while MS-FBG sensors can provide temperature-independent pressure measurements without a requirement for an extra FBG. Since Bi-FBGs inherently have a high PDFS, any polarization changes will dominantly affect the amplitude of the FBG peaks rather than their position.
EP Patent publication number EP 1192500, assigned to Optoplan, discloses a method for measurements of the orthogonally polarized minimum and maximum Bragg wavelengths of one or several birefringent fibre Bragg grating FBG sensors. However a problem with gathering such information from Bi-FBGs is that high precision monitoring and interrogating of their polarization-dependent wavelengths in order to provide sensing applications with an acceptable accuracy at a high acquisition speed is required.
It is therefore an object of the invention to provide a measurement system and method to interrogate and monitor birefringent optical sensors.
It is a further object of the invention to reduce the complexity of the system while maintaining high speed and resolution measurements.